1. Field of the Invention
The present invention relates to a sheathed heater and, more specially, to a sheathed heater having improved durability.
2. Prior Art
A conventional sheathed heater comprises a metallic sheath, a coiled metallic heating element sheathed in the metallic sheath and an insulating material, such as magnesia powder (MgO powder) filled in the sheath to insulate the metallic heating element from the metallic sheath and to insulate the coils of the metallic heating element from each other. A sheathed heater employing boron nitride and magnesia as insulating materials (filling material) is disclosed in Japanese Utility Model Application No. 57-52871.
In such a conventional sheathed heater, the oxygen component of the filled magnesia is liable to dissociate and to oxidize the metallic heating element gradually until the heating element is broken, particularly in the course of a long period of use.